The selective affinity of M.gallisepticum (S-6 strain) for the wall of arteries in the CNS and periarticular tissues, demonstrated by immunofluorescence, will be studied further in chickens and turkeys. In addition, chick embryos will be used in order to see whether selective localization in vessels can be established. Human periarticular tissues, from routine diagnostic synovial specimens, will be employed for immunofluorescence with antisera directed against various known mycoplasma species. Membrane-bound particles resembling mycoplasms have been observed in high-speed sediments of broth cultures of rheumatoid arthritis joint fluid. Further studies of these are planned.